Copper-containing catalysts have wide application in chemical technology. Depending on the specific use to which they are put, they differ primarily in the materials which they contain and the quantitative composition thereof. Specifically, copper oxide/zinc oxide/aluminum oxide catalysts are the basis of modern processes for synthesizing methanol from carbon monoxide and hydrogen at low pressures.
For instance, DE-A-20 56 612 describes catalysts for preparing methanol which are compounds of the mixed crystal series (Cu.sub.x Zn.sub.y)Al.sub.2 (OH).sub.16 CO.sub.3.4H.sub.2 O, where x and y are 0.5 and 5.5 and the sum of x and y is 6. The desired mixed crystals are obtained by reaction of aqueous solutions containing copper nitrate, zinc nitrate, and aluminum nitrate with basic reagents, such as aqueous sodium carbonate solution, at pH values of 4.5 to 5.5.
EP-A-01 25 689 also relates to copper oxide/zinc oxide/aluminum oxide catalysts for methanol synthesis. They are characterized by a Cu/Zn atomic ratio of 2.8 to 3.8 and an Al.sub.2 O.sub.3 content of 8% to 12% by weight. They are prepared by coprecipitating copper and zinc with alkaline materials, such as alkali metals or ammonium carbonate, in the presence of the aluminum oxide component, preferably colloidally dispersed aluminum oxide or hydroxide, from their aqueous solutions. In the unreduced catalyst, 20% to 40% of the pores have a radius of from 1.0 to 3.75 nm and from 60% to 80% of the pores have a radius greater than 3.75 nm.
A further application for the above compositions is hydrogenation of organic compounds. For this type of reaction, CuO/ZnO/Al.sub.2 O.sub.3 catalysts replace copper/chromium oxide catalysts (known as Adkins catalysts), the use of which has been avoided in recent times for ecological reasons, and they are in competition with the multiplicity of nickel catalysts which have been described and introduced to industry.
Hydrogenation catalysts based on copper oxide, zinc oxide and aluminum oxide wherein at least about 80% of their pore volumes consists of pores having a diameter greater than about 80.ANG. (8 nm) are the subject of EP 04 24 061. In preferred embodiments, the catalyst powder has a surface area of at least 70 m.sup.2 /g, the average particle diameter is about 8 to about 28 .mu.m and the atomic ratio of copper to zinc is about 0.2 to about 5.5. The catalysts are produced by preparing two aqueous solutions, one containing copper and zinc salts and the other containing a basic aluminum salt (for example sodium aluminate) and a basic precipitation reagent (for example soda). The two solutions are mixed with one another in a ratio such that the pH of the resulting mixture is at least about 7. The precipitated solid is subsequently filtered off and calcined. The catalyst is used for hydrogenating aldehydes, ketones, carboxylic acids, and carboxylic esters.
The known catalysts containing copper oxide, zinc oxide, and aluminum oxide have frequently been developed specifically for particular reactions; e.g. methanol synthesis or the hydrogenation of organic compounds. They can nevertheless also be used for other chemical reactions but, in such cases, do not always produce optimal results. Even different ways of carrying out the same reaction may require the provision of individualized catalysts. Experience has shown that the reaction of the same reactants under different conditions (for example in gaseous or in liquid form, on fixed catalysts, or on a catalyst suspended in the substrate) requires specially adapted catalysts. In this context it must be remembered that, in processes which are carried out on a large scale, an increase in conversion or an improvement in the selectivity in the order of even a few tenths of a percent can lead to significant economic advantages.